The spin rate of golf balls is the end result of many variables, one of which is the distribution of the density or specific gravity within the ball. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers. High spin rate allows the more skilled players, such as PGA professionals and low handicapped players, to maximize control of the golf ball. A high spin rate golf ball is advantageous for an approach shot to the green. The ability to produce and control back spin to stop the ball on the green and side spin to draw or fade the ball substantially improves a player's control over the ball. Hence, the more skilled players generally prefer a golf ball that exhibits high spin rate, in part, off scoring irons, such as the 7-iron club through the pitching wedge.
On the other hand, the recreational players who cannot intentionally control the spin of the ball generally do not prefer a high spin rate golf ball. For these players, slicing and hooking the ball are the more immediate obstacles. When a club head strikes a ball improperly, an unintentional side spin is often imparted to the ball, which sends the ball off its intended course. The side spin reduces a player's control over the ball, as well as the direct-line distance the ball will travel. A golf ball that spins less tends not to drift off-line erratically if the ball is not hit squarely with the club face. A low spin ball will not cure the hook or slice, but will reduce the adverse effects of the side spin. Hence, recreational players typically prefer a golf ball that exhibits low spin rate.
Reallocating the density or specific gravity of the various layers of a golf ball provides an important means of controlling the spin rate. In some instances, the weight from the outer portions of the ball is redistributed toward the center to decrease the moment of inertia, thereby increasing the spin rate. For example, U.S. Pat. No. 4,625,964 discloses a golf ball with a reduced moment of inertia having a core with specific gravity of at least 1.50 and a diameter of less than 32 mm and an intermediate layer of lower specific gravity between the core and the cover. U.S. Pat. No. 5,104,126 discloses a ball with a dense inner core having a specific gravity of at least 1.25 encapsulated by a lower density syntactic foam composition. U.S. Pat. No. 5,048,838 discloses another golf ball with a dense inner core having a diameter in the range of 15-25 mm with a specific gravity of 1.2 to 4.0 and an outer layer with a specific gravity of 0.1 to 3.0 less than the specific gravity of the inner core. U.S. Pat. No. 5,482,285 discloses another golf ball with reduced moment of inertia by reducing the specific gravity of an outer core to 0.2 to 1.0.
In other instances, the weight from the inner portion of the ball is redistributed outward to increase the moment of inertia, thereby decreasing the spin rate. U.S. Pat. No. 6,120,393 discloses a golf ball with a hollow inner layer with one or more resilient outer layers, thereby giving the ball a soft core, and a hard cover. U.S. Pat. No. 6,142,887 discloses an increased moment of inertia golf ball comprising one or more layer layers made from metals, ceramic or composite materials, and a polymeric spherical substrate disposed inwardly from the layer layers.
The redistribution of weight within the golf ball is typically accomplished by adding fillers to the inner core or to an outer layer of the golf ball. Conventional fillers include the high specific gravity fillers, such as metal or metal alloy powders, metal oxide, metal searates, particulates, carbonaceous materials, or low specific gravity fillers, such as hollow spheres, microspheres or foamed particles. However, the addition of fillers may adversely interfere with the inherent resiliency of the polymers used in golf balls and thereby the coefficient of restitution of the golf balls. Hence, there remains a need in the art for a golf ball with controlled moment of inertia that has improved coefficient of restitution.